Capacitors containing an electrolyte solution are subject to failure caused by leakage of the electrolyte liquid or vapor. For example, it is common for gas, such as hydrogen, to be evolved during operation, causing pressure to build inside the capacitor. Consequently, leaks may occur around conventional non-hermetic polymeric seals, where the terminal, for example a wire, protrudes from the capacitor housing.
To avoid the leakage, a gas-tight hermetic seal is required. One prior art solution has been to provide a hermetic, outer metal-glass-inner metal seal. The “inner metal” component of the hermetic seal is typically an electrically conducting post, which is connected to the terminal wire and electrically insulated from the housing by the glass component. The “outer metal” component of the hermetic seal may be an annular band around the glass, which is bonded to the housing. Alternatively, the hermetic seal may be cast in an opening that has been created in the housing, so that the housing itself forms the outer metal component of the hermetic seal.
In the situation of an aluminum case, it has not proven economical, however, to provide a hermetic, aluminum to glass seal, due in part to the significant difference in the thermal coefficient of expansion of the glass or ceramic material used to construct the seal and the thermal coefficient of expansion of aluminum. Consequently, the hermetic seals used in capacitors are generally made with a metal other than aluminum, for example, steels (stainless or other alloys) or tantalum. Although the outer metal portion of the hermetic seal is at approximately the same potential as the electrolyte, in general, if the outer metal is welded to the housing, it should be of the same material as the housing in order to avoid galvanic corrosion, if this region is exposed to the ionically conducting electrolyte.
To avoid galvanic corrosion of the inner-metal portion of the hermetic seal, a liquid-tight seal is typically utilized to prevent exposure of the inner region of the hermetic seal to the electrolyte. Even in the most optimum situation in which all components along the electrically conducting path between the inner-metal portion of the hermetic seal and the anode of the capacitor element consist of the same “valve metal”, so that galvanic corrosion is not an issue due to a dissimilar metal junction, a liquid-tight seal is still used to prevent the electrolyte from making contact with the hermetic seal.
A “valve metal” is defined as a metal which grows an electrically insulating oxide in the presence of an electrolyte when a positive potential is applied to the metal with respect to the electrolyte. Examples of such metals are aluminum, tantalum, niobium, tungsten, titanium, zirconium. The two primary reasons that the inner portion of the hermetic seal, including a valve metal seal, should be protected from the electrolyte are the possibilities of intermetallics (impurities) in the valve metal that may not form a proper electrically insulating oxide in the presence of an electrolyte and/or insufficient creepage distance across the glass portion of the seal. Either of these two situations could result in undesirable electrical current flow between the high electrical potential of the inner-metal and the low potential of the outer-metal if the electrolyte is allowed access to all regions of the hermetic seal.
Various capacitor constructions have been disclosed to protect the components of the hermetic seal from corrosion. Sloan, U.S. Pat. No. 3,289,051, discloses a threaded cap containing a hermetic seal, which is screwed on the lid of the capacitor to compress a stack of bushing members. The apparatus of Sloan is complex to manufacture, requiring the assembly of numerous components, many of which must be welded together to maintain the hermeticity of the capacitor. Further, the means to seal the capacitor adds substantial bulk and protrudes from the top of the capacitor.
A paper entitled “Longlife, High-Voltage, Hermetically-Sealed Aluminum Electrolytic Capacitors,” presented at CARTS 96: 16th Capacitor and Resistor Symposium, 11-15 Mar. 1996, discloses a hermetic seal. The seal is contained in a multi-component cap welded to the lid of the capacitor. In particular, a metal-glass-metal hermetic seal is welded to a stainless steel tube or cylinder, which is in turn welded to an aluminum cup having a liquid tight seal in the base of the cup. The aluminum cup is welded to the lid of the capacitor. In addition to requiring specialized aluminum to stainless steel welding techniques and multiple welds, the cap is bulky and protrudes from the top of the capacitor.
In U.S. Pat. No. 4,987,519, Hutchins et al. disclose a cylindrical capacitor with a seal created by crimping. An inwardly directed annular bead is formed, which presses an O-ring into a plastic bushing. While the foregoing technique has found utility with cylindrical capacitors, it is not effective for sealing capacitors having other geometric configurations, such as a rectangular prism. Additionally, the capacitor disclosed in U.S. Pat. No. 4,987,519 requires a second seal where the riser wire protrudes from the plastic bushing.
Capacitors having a non-cylindrical casing, especially capacitors having a casing with a flat surface, are particularly difficult to seal. Prior art methods of sealing the liquid typically employ a gasket or seal around the inside perimeter of the casing. As gas pressure builds inside the capacitor, the flat surface may bulge outward, creating a gap between the O-ring or gasket and the casing. Electrolyte can seep through the gap and corrode the terminal in the hermetic seal.